The present invention generally relates to an apparatus and method for detecting a depolarization activation wave (R wave) of the heart. The present invention is more particularly directed to a fully automatic atrial defibrillator which employs automatic gain control to assure reliable detection of depolarization activation waves of the heart.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life-threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness or even loss of consciousness.
Atrial fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, implies applying cardioverting or defibrillating electrical energy to the heart in synchronism with the detected depolarization activation wave (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistant to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide relief to patients suffering from occurrences of atrial fibrillation. Unfortunately, to date, none of these atrial defibrillators have become a commercial reality, to the detriment of such patients.
Implantable atrial defibrillators proposed in the past have exhibited a number of disadvantages which probably have been the cause of these defibrillators failing to become a commercial reality. Two such proposed defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these defibrillators require the patient to recognize the symptoms of atrial fibrillation with one defibrillator requiring a visit to a physician to activate the defibrillator and the other defibrillator requiring the patient to activate the defibrillator from external to the patient's skin with a magnet.
Synchronizing the delivery of the defibrillating or cardioverting electrical energy with a depolarization activation wave (R wave) of the heart is important to prevent ventricular fibrillation. Ventricular fibrillation is a fatal arrhythmia which can be caused by electrical energy being delivered to the heart at the wrong time in the cardiac cycle, such as during the T wave of the cycle. As a result, it is most desirable to sense depolarization activation waves of the heart to generate synchronization pulses (or signals) in a manner which avoids detecting noise as a depolarization activation wave. Also, it is desirable to sense depolarization activation waves in a manner which does not fail to detect extremely low amplitude depolarization activation waves but yet does not erroneously detect low amplitude noise such as T waves.
In addition to noise, a further problem in reliably detecting depolarization activation waves is the variability of the amplitudes of depolarization activation waves. Particularly during atrial fibrillation, depolarization activation waves are characterized by extreme variability. An extremely low amplitude depolarization activation wave may be followed immediately by an extremely high amplitude depolarization activation wave.
Depolarization activation waves are generally sensed as voltage threshold crossings. An atrial defibrillator includes an input sense amplifier in electrical contact with a patient's heart for sensing electrical activity of the heart. The sense amplifier amplifies the electrical activity according to a predetermined gain. A depolarization activation wave is detected when the amplified electrical activity exceeds a predetermined threshold.
Because of the variability in depolarization activation wave amplitude, some depolarization activation waves may be too small in amplitude to exceed the detection threshold of prior atrial defibrillators, meaning these depolarization activation waves may not be detected. Merely increasing the gain of the input sense amplifier which senses electrical activity of the heart does not provide a solution, since high amplitude depolarization activation waves may saturate the sense amplifier and provide sensing of undesirable signals such as the T wave. The apparatus and method of the present invention provides an approach to reliable detection of depolarization activation waves which accommodates extreme variability in depolarization activation wave amplitude.